Inadequate solder volume commonly occurs when through hole components/connectors are soldered using a pin-in-paste process. Solder paste deposit height may be limited by the use of thinner stencils (5 mils or less) to eliminate solder balls or slumping in fine pitch applications. Using overprinting (apertures larger than the plated through hole annular ring) to provide additional solder volume often results in bridging or random solder balls. Using thicker stencils (8 mils or more) is an option, but this cannot meet the solder volume requirements in certain cases, such as when thick boards (0.092″ or greater) are used, or when pins with rectangular cross sections are inserted into round through holes.
Square leads soldered into round plated through holes are a common cause of a solder paste volume deficit in pin-in-paste processes. For example, when soldering a square pin with 0.025″ sides into a 0.038″ round hole (the smallest round hole that this pin fits into) using a thick stencil (0.010″), substantial overprint (200%), a thin board (0.062″), high transfer efficiency of the paste (100%), and no paste drippage from the bottom of the hole, only 56% of the volume between the pin and barrel sidewalls defining the hole will be occupied by solid solder after reflow.
The problem of inadequate solder volume in SMT processes has been addressed by the use of solder preforms. Solder preforms are precise metal shapes which provide a highly repeatable volume of solder. In contrast to solder paste, solder preforms are 100% metal content by volume. They are used in conjunction with solder paste to incrementally increase the volume of solder joints, which increases reliability in connections subject to mechanical fatigue, and increases signal-to-noise ratios in interconnections delivering high frequency signals. Preforms are produced by high-speed stamping or forming from a solder wire or ribbon.
In assembling components using solder preforms, solder preforms are picked from a solder preform holder (typically a tray or reel of tape formed with pockets for holding the preforms) by a pick and place apparatus of the type that is known in the art for placing chips on boards, such as available from Panasonic Factory Automation Company of Elgin, Ill. under the model numbers BM221 CM202, CM 301 and others. Each preform is presented to a vacuum nozzle which picks the preform up by application of vacuum when the nozzle is proximate the preform. The nozzle then moves to a predetermined position over a substrate and the vacuum is interrupted or at least reduced in strength sufficiently to release the preform and place it in its proper position on the substrate. High error rates can result if there is not a well sealed vacuum between the nozzle and the preform, because the nozzle will not pick up the preform, or it will release the preform prematurely.
One cause of a poorly sealed vacuum is curvature at corners or edges of the top or bottom surface of a preform. This curvature can be imparted by forces acting on the preform as it is stamped from metal stock. If this area of curvature occupies a substantial fraction of either the top or bottom surface of a preform, the margin for error for the vacuum nozzle is drastically reduced. Poorly sealed vacuum can also occur if the top and bottom surfaces of the preform are not substantially parallel.